The majority of the energy used today is derived from fossil fuels, despite the on-going controversy surrounding their environmental impact. Fossil fuels, as with any carbon-containing materials, release carbon dioxide upon their combustion. The extraction of fossil fuels for energy production results in the release of carbon into the atmosphere that was previously stored in the earth, and thereby has a net effect of increasing the levels of atmospheric carbon dioxide. A major source of atmospheric fossil carbon dioxide comes from “tailpipe emissions” from cars and carbon dioxide-containing flue gases from fossil fuel burning power plants.
On the other hand, carbon dioxide released from combusting fuel derived from non-fossil organic material is relatively benign, given that it simply returns to the atmosphere carbon that was recently fixed by photosynthesis. More generally, this relatively benign nature is also true of carbon dioxide released as a byproduct from the processing of non-fossil organic material during fermentation or other processes that break down organic material into simpler molecules. Carbon dioxide sourced from non-fossil organic material is referred to herein as biogenic carbon dioxide, as described below. Fuels or fuel intermediates containing biogenic carbon are known as “biofuels” or “biofuel intermediates” and the tailpipe emissions from biofuels are generally considered benign to the atmosphere.
Displacing fossil-based fuel with fuel made from non-fossil organic material creates atmospheric greenhouse gas (GHG) benefits by displacing carbon dioxide emissions that would have been from the fossil fuel and would have led to an increase in atmospheric levels of carbon dioxide. Carbon dioxide is a greenhouse gas and has been identified as a contributor to global climate change. Various governments have promoted the increased use of renewable fuel through legislative and regulatory regimes, including the Energy Independence and Security Act (EISA) in the U.S. Some of the purposes of the EISA are to increase the production of clean renewable fuels, to promote research on and deploy GHG capture, and to reduce fossil fuels present in transportation fuels. In addition to EISA, numerous jurisdictions, such as the state of California, the province of British Columbia, Canada and the European Union, have set annual targets for reduction in average life cycle GHG emissions of transportation fuel. Such an approach is often referred to as a Low Carbon Fuel Standard (“LCFS”), where credits may be generated for the use of fuels that have lower life cycle GHG emissions than a specific baseline fuel.
Despite these government incentives, biofuels still do not enjoy widespread use due to technical and cost limitations. One challenge with commercializing biofuels is that the yield of fuel from the starting material is often low. A variety of factors contribute to these low yields. For example, in the fermentative production of ethanol from non-fossil organic material, such as corn, a significant amount of the carbon from the sugar is not converted into fuel product. During fermentation, the yeast produces carbon dioxide in addition to the desired ethanol product. From one mole of glucose, two moles of each ethanol and carbon dioxide are produced. This carbon is usually not captured as the carbon dioxide is typically vented to atmosphere due to its low energy value and, given that the carbon dioxide is biogenic, it has no net effect on the life cycle GHG emissions of the ethanol. Another issue is the cost and GHG emissions associated with purifying the ethanol product.
Moreover, only the carbohydrate-rich portion of organic material, such as grain or the stalks of sugar cane, is readily converted to ethanol. While the production of fuel from these parts of the plant can be carried out with relative ease, the structural parts of the plant also contain sugar in the form of cellulose and hemicellulose, which is generally more difficult to convert to biofuel. Since these parts of the plant are not converted to product in such fuel fermentation processes, this represents a significant yield loss.
Research efforts have been directed toward the development of processes that can convert the non-edible cellulose and hemicellulose portion of plant material to fuels. A first chemical processing step for converting non-edible parts of plants to ethanol, or other fermentation products, involves breaking down the fibrous material to liberate sugar monomers from the plant material. This can be achieved by hydrolyzing the hemicellulose first to its constituent sugars, using a chemical such as sulfuric acid, followed by hydrolysis of the cellulose to glucose by enzymes referred to as cellulase enzymes. These sugars are then fermented to ethanol with yeast or bacteria. A non-sugar containing component that remains after the conversion, known as lignin, can be burned to generate heat and power for internal plant operations. Thus, the process benefits from maximizing the whole plant for fuel or energy production. Nonetheless, there are challenges in obtaining a high yield of sugar for subsequent fermentation due to the recalcitrance of the cellulose to enzymatic hydrolysis. Although there is on-going research aimed at improving the efficiency of this step, progress is slow and the process is still costly.
Another approach for utilizing the whole plant involves subjecting the organic material to gasification to make syngas, which is composed of carbon monoxide, hydrogen and typically carbon dioxide. Syngas can then be used as a precursor to make additional chemicals or used as a fuel itself. While the whole plant, including both the carbohydrate and lignin components, can be converted to syngas, some of the energy stored in the sugar polymers is lost in the process. Moreover, many side products are produced, including tars and carbon dioxide which are not converted to fuel and thus contribute to yield loss.